Convert categories

Given the issues of GtkComboBox with GTK4 found after the GTK4 migration and the lack of search support reported in #108 (closed) I made the following changes:

• added a unit category dropdown above the unit selectors
• replaced the unit selector GtkCombos with GtkDropDowns
• to tackle #108 (closed) I added search support for dropdowns whenever there are more than 10 items in the unit selectors

Given that the conversion UI needs a refresh, but we don't want to block GTK4 support (already done in !98 (merged) if this is accepted from design point of view by @bertob @exalm @jimmac), the above seems like a good solution until we get to redesign the conversion UI.

If the screenshots are not enough, the code can be tried out using GNOME Builder via Flatpak (that's the way it was developed) so feel free to play around.

Attaching a quick screenshot of the views using the converter from the GTK4 Build :

• the Financial (the unit category selector is hidden, unit selectors showing currencies by default, as we had in the original combos)

• the Advanced UI, with the category selector visible