LocationBar: Use GtkStack to manage three independet widgets for search/path-entry/breadcrumbs

Original Title: It's possible to start searching by typing "Search: " in the Location bar, but search mode is not entered

Version: Thunar git main as of 909efefe.

Steps to reproduce

As the title says, it is possible to start a new search by pressing Ctrl+L (Open Location…) and entering a string such as "Search: some search terms". However the display isn't the usual search listing: if you are in Icon View to start with, the search results are shown in Icon Vew too:

Click to expand screenshot

This "search" cannot be terminated with Esc, and the search togglebutton on the toolbar is not toggled on.

Continued weirdness

If you now press the more conventional Ctrl+F combination (Search for Files…), then a new "real" search begins. It's as if Thunar wasn't in search mode at all before, despite the search results clearly showing!

Click to expand screenshot

The results are shown in List View, which is what I expect, and now the Search button is toggled on. This search can be terminated with Ctrl+F or Esc while the search entry box has focus and Thunar will switch back to Normal mode.

Observations

Thunar should enter "proper search mode" no matter which keys you press to initiate the search. This becomes important if one fix for #787 (closed) would be to stop monitoring while in search mode.

I don't like the search entry box being actually the location entry box with some editable text stuffed in the front somehow. It seems to lead to situations like this when you consider that strings have to be parsed. I think I'd prefer two separate boxes in a GtkStack whose state gets toggled when you press Ctrl+F, Ctrl+L, or Esc. If there's to be text at the front of the search entry box, make it static and uneditable so that I can press Ctrl+A to go to the start of the search string like a normal text entry box.