Summary:
1) Now the selector has *no* selection if the currently active preset
   is not present in the filtered list of presets
2) Clicking on the currently selected preset now either resets it,
   or does nothing depending on the "Save tweaks..." option value.
3) All the three preset selectors are now synchronized
4) The selector correctly shown the "dirty" mark right after you
   changed any property of the preset.

Test Plan:
1) Check sanity of the current selection in all the three preset
   selectors. They should be sane and synchronized.
2) Switching the current tag should keep the selection correct (no
   selection is a valid state when the active preset is not present
   in the current tag)
3) (!) Check if Brush Selector, Texture Selector, Pattern and Gradient
   popups still work correctly! They use the same codebase :(

Ref T3473

Reviewers: #krita, timotheegiet

Maniphest Tasks: T3473

Differential Revision: https://phabricator.k...