[TSM] Avoid duplicating session actions on re-save.

Fixes #1782.
[TSM] Better code.
[TSM] SessionsManager::deleteCustomSession() for #1783.
[TSM] "Delete session" actions for #1783.
[TSM] Reworked load/delete session handlers with SlotClosure.
[TSM] Remove session from menus on delete, closes #1783.
[TSM] Enable/disable submenus.
[TSM] Renamed signal sessionRequested -> loadRequested.
[Util] Code deduplication.