Add "Copy Current Filter" action

[bmatherly]

As suggested here:
https://forum.shotcut.org/t/copy-single-filters/13909

This PR adds a new button next to the existing "Copy Filters" Button. This new button is called "Copy Selected Filter".

In the forum discussion, the conversation converged on the idea of adding multi-select to the filter list so that the user can choose what filters to copy. I am in support of this. However, I only implemented single filter copy in this pull request. The reasons that I did not implement multi-select are:

1. The QML ListView Item does not offer multi-select. So I added a lot of extra logic to disable the built-in selection and to implement my own multi-select implementation on top of it. It was mostly working.
2. After I implemented multiselect for the list view, I found that there are a lot of dynamic interactions between the QML filter list, filter controller, and filter dock. I kept adding more and more code to keep all three classes happy with the multi-select.

Eventually, I decided that the extra complexity and risk of regression is too great and I changed to this simpler single selection implementation.

[ddennedy]

I am not a fan of the icon and adding a confusing alternative copy action next to the other on the tool bar. Perhaps the existing copy should have a drop down arrow to choose from alternatives:

• Enabled (checked)
• All
• Selected

You must click the drop down arrow to access the alternatives (click icon chooses Enabled) unless you use the actions window or assign a shortcut. What do you think of that proposal?

[bmatherly]

I agree with your comments. Unfortunately, the QML button Item does not have all the same features as a QButton and does not offer the dual function of an action and a submenu.

I submitted a variation that always shows the submenu with the three options.

I think that maybe it looks a little strange that they all have the same icon. Maybe I should just remove the icon.

[ddennedy]

Please remove the icons from the submenu. Do we need the “filters suffix?” It should appear in the “Filters >” group in the Actions window. The singular vs. plural might raise issues from translators, and the plural form of the tr method needs to supplied with a count parameter. Otherwise, I’m fine that it always opens the menu.

[bmatherly]

Both good comments. This is done. You were right - the Shortcuts menu still makes sense because it says "Filters -> Copy All", etc.