This builds on the scrolling mode work to add three buttons for joining
page spreads together: one for the default view, with no page spreads,
and two for spreads starting on odd-numbered or even-numbered pages.
Please note that this patch *purposely* doesn't add every standard (or semi-standard) page name in existence, but rather only a few common ones. This is done to lessen the burden on localizers, since it's quite possible that all of the page names could need translation (depending on locale).
It's easy to add more standard page sizes in the future, but we should take care to *only* add those that are very commonly used in actual PDF files.
The units are currently repeated after each dimension, which seems unnecessary and is also not done in other PDF viewers (such as e.g. Adobe Reader).
Furthermore, the name of the l10n arguments can be simplified slightly, since the name of the strings themselves should be enough information.
Finally, the `width`/`height` should be formatted according to the current locale, as is already done for other strings in the document properties dialog.
This required changing the import script in two ways:
- we should use the `default` branch and not the `tip` tag since the
latter may refer to another branch than `default` (this is the case for
the `vi` locale, which caused in the files to be overwritten with
incorrect contents since `tip` referred to the
`THUNDERBIRD600b1_2018031614_RELBRANCH` branch);
- we should check if the response code is indeed 200 because recently a
script removed all empty localization files upstream (refer to
https://bugzilla.mozilla.org/show_bug.cgi?id=1443175).
With the current way that the `HandTool` is implemented, if someone would try to also add a Zoom tool (as issue 1260 asks for) that probably wouldn't work very well given that you'd then have two cursor tools which may not play nice together.
Hence this patch, which attempts to refactor things so that it should be simpler to add e.g. a Zoom tool as well (given that that issue is marked as "good-beginner-bug", and I'm not sure if that really applies considering the current state of the code).
Note that I personally have no interest in implementing a Zoom tool (similar to Adobe Reader) since I wouldn't use it, but I figured that it can't hurt to make this code a bit more future proof.
A longstanding issue with the viewer is that you cannot tell if a PDF document includes an outline and/or attachments without actually opening the sidebar.
This patch contains a suggested solution for that, by displaying an hide-on-interaction notification on the `sidebarToggle` button (and the relevant sidebar view buttons). Note that this was inspired by e.g. the update notification that is displayed on the menu button in Firefox.
For an initial implementation, I've tried to do this in such a way that the notification isn't too distracting. Without being an UX expert, I don't think that we'd want something too in-your-face, in order to keep the viewer toolbars reasonable clean. (We probably do *not* want e.g. an entire notification bar in these situations, since that would take up unnecessary screen space and require actions from the user to close.)
However it's certainly possible that the current notification might simply be *too* inconspicuous to be truly helpful to users, but we could probably iterate on that if the feature itself is deemed useful.
