Add session backup and restore

Closes #21 (closed).

This MR is probably a first step towards #3 (closed), or more generally the ability for Mousepad to recover from a crash, an INT signal, etc.

#32 (closed) will probably be addressed at some point during these developments.

A "File" tab is added to the prefs dialog with a setting for this MR, and (some of) those added in !64 (merged), !69 (merged) and !89 (merged) (extended in !107 (merged)):

When the recent file menu length reaches 0, the "Clear History" dialog opens to ask the user for confirmation before clearing the recent history.