Refactor Tools menu to support ordering, promotions and separators

Currently the Tools menu is sorted alphabetically. Two hacks are implemented:
1) Leading spaces can be used to adjust the ordering. The sorting is done on the full menu name, but then the leading spaces are removed.
2) An exclamation mark as the first symbol can be used to promote a Tool menu to the menu bar. This exclamation mark is removed before displaying the menu name.

This can be improved by:
1) Explicitly introducing the menu priority as an integer. Within the same priority the items can still be sorted alphabetically.
2) Adding a boolean flag to promote an item to the Menu bar.

Additionally a possibility to group tools into sub-menus is needed. For example, the items "Tools->Merge selected places" and "Tools->Merge selected transitions" would look better as "Tools->Merge->Selected places" and "Tools->Merge->Selected transitions" respectively.