Multi select masks

[bnmajor]

This PR:

• Rearranges the buttons to try to provide a more intuitive layout
• Add support for multi-select in the tree view (CTRL + left-click to select individual items, SHIFT + left-click at start and SHIFT + left-click at end to select all items in between)
• Can use CTRL + a to select all
• Allows changing the presentation of multiple masks at once
  • If more than one row is selected the combobox on the right will be enabled
  • If one or more of the selected masks are powder, laue, or threshold the options will be limited to None or Visible. Otherwise the Boundary only and Visible + Boundary will also be options.
  • The changes are applied to all selected masks immediately
• Adds a button to export selected masks
  • This is functionally the same as selecting multiple rows and using the context menu to export, just provides the option of a button if that is preferred over right-click
  • The button is only enabled when more than one row is selected
• Adds the option to remove multiple masks at a time

• Relies on New masking features #1815, Allow changing line style and width for mask borders #1848

Fixes #1846

[psavery]

I wonder if we can just get rid of the "Remove Mask" buttons on each row, and rely on the "Remove Selected" instead. What do you think, @saransh13?

Overall, the code changes look good to me! Let us know what you think, Saransh.

[psavery]

I would typically do something like this:

        enable = len(selected) > 1
        widgets = [
            self.ui.presentation_selector,
            self.ui.export_selected,
            self.ui.remove_selected,
        ]
        for w in widgets:
            w.setEnabled(enable)

But you do not have to change it!

[saransh13]

I'm approving the PR but will note that the mask to panel buffer is not working as expected. Since this is also a problem in master, we can fix that issue in a follow up PR.