Tool window contents are not properly disposed

[kpatenio]

The tool window doesn't seem to be properly disposed once a merge conflict is resolved or aborted. This can potentially lead to memory leaks if the IDE is used for a long duration. Will need to also document how we should dispose of certain objects.

Example 1

ConflictRegion instances from file1 are kept in memory whenever open file2, despite their nodes not being displayed on the tool window. The tree then instantiates ConflictRegion instances for the file2 and displays them. Once a merge conflict is completely resolved or aborted, the ConflictRegions still remain in memory.

Example 2

After aborting or resolving a merge conflict, an instance of the tool window remains in memory. Upon triggering the merge conflict again, a new tool window instance is created while the previous one still remains in memory.