Improve support when specifying a pre-existing toolbar as the target of agw.aui Minimize

[jmoraleda]

agw.aui includes the option to specify a pre-existing toolbar as the location where the restore icon/tool should go when minimizing panes -- instead of the default behavior which is creating a new automatic toolbar.

But the option of specifying a pre-existing toolbar cannot handle loading new perspectives or closing or detaching minimized panes. The main issues when specifying a pre-existing toolbar are:

• Using the tool id to find the tool does not work well because multiple tools with the "ID_RESTORE_FRAME" id could have been added to the same toolbar
• Using the pane label/caption to find the tool does not work well when loading a perspective because the caption of the pane could be changed by the load operation
• When loading a perspective, a pane that was minimized could become unminimized or vice versa, and when using a pre-existent toolbar, the toolbar would be preserved by the perspective change operation and the restore icon would need to be added or deleted from the toolbar as appropriate, but not added twice if it was already there.

This PR addresses these problems.

[jmoraleda]

Rebased to latest master, since it did not apply cleanly after merging #2494

[Metallicow]

It is still a problem with agw.aui, but it seems you have collapsed two of the issues into a single subject.
These issues may be subject to review.
Look into the depth....