[Feat] Add New Mapset action to the Data catalog

[wenzeslaus]

Is your feature request related to a problem? Please describe.
Although, mapsets can be already created from the Startup window and main menu in GUI, the Data catalog (Data tab, Data tree) should be the central place to manage data, so it should have an equivalent of the "new mapset" button from the current Startup window.

Describe the solution you'd like
Add a New mapset action to the context menu of Location tree items in the Data catalog. In other words, user should be able to go to the Data tab, right click on current location node, select "New mapset" from the menu to create a new mapset.

Discussion
The following does not have to be included in the initial implementation (PR), but if people are interested, we can discuss: Should new mapsets be created only in the current location as in the current GUI (main menu) or in any location (like in the Startup and possibly more fitting to the Data tab)? How "allow editing other mapsets" (lock/unlock button) plays into this? Should the GUI switch automatically or based on additional input to this new mapset? If so, how? Additional item in the context menu? Two buttons "Create" and "Create and switch" in the dialog? Should the "New mapset" context menu entry appear for location nodes in the tree or also for mapsets (or even its children) with wording like "Create new mapset here" (so user does not have to find the location node to create another mapset "next" to the current one)?

Additional context
Same functionality is already in the Startup window (New button for mapset) and in Settings > GRASS working environment > Create new mapset, so it can be just reused (called) here.

Migrated from Trac ticket 3579.