Clean up undo handlers when closing a tab (closes bgo#678048)

When the undo handling mechanisms were moved around in commit
c8eb4d65c0bbbf3168250a2672c1f786e650cd67 the tab closing case wasn't
dealt with.

While tab removal does trigger a switch-out, and could theoretically
be handled in the switch-page handler, there's no way to get at the
page being switched from if that page has been removed. Therefore, we
add a simple clean up of the signal handlers in try_remove_page.
Introduce newly-written Mallard-based help
Fix table markup on keyboard shortcuts page
Explain why the "Things that Meld doesn't do" page exists
Add help section on saving file changes
Use 'change' instead of 'chunk' as a more understandable term
Remove rarely-used "Hide" and "Case" toggles from the toolbar

This commit removes the "Hide" and "Case" (case insensitive comparison)
toggles from the toolbar. This is mostly because they're not a common
use case, and so don't belong there, but also because they're not
sufficiently easy to explain, and have no commonly recognisable icon.
Change case insensitive comparison description to make some sense

When writing documentation for this, I couldn't bring myself to write
the sentence "You can do this by clicking View->Case", so changed the
label.
Added explanation and examples of file filtering and case sensitivity
Fill out file filtering documentation
Fix example comparison listing in text filter docs
Add most of the folder mode documentation
Small wording changes
Fix the colour description of some states in folder mode
Reorder some VC pages
Document version control mode and associated options
Add credit and license information to every help file
Fix internal links and file naming issues
Updated Polish translation
Updated Slovenian translation