Fix memory leak caused by stale link tags in GuiGramplet

[ztlxltl]

How to reproduce the leak:

1. Open the example database.
2. Switch to the people list view.
3. Add the Pedigree Gramplet to the sidebar.
4. Open the config of the Pedigree Gramplet and make sure the "Max generations" is set to 100.
5. Select Warner, Allen Carl.
6. Move the mouse over the names in the pedigree.
7. The names are underlined while the mouse pointer is hovering over them.
8. Select Warner, Amanda.
9. Repeat switching between Warner, Allen Carl and Warner, Amanda about 20 (loading the pedigree of both 10 times).
10. Move the mouse again over the names in the pedigree.
11. The underlining appears delayed.
12. Repeat switching between Warner, Allen Carl and Warner, Amanda a few dozens of times, depending on your machine.
13. Updating the pedigree becomes slower and slower.

The reason for the delays is that PedigreeGramplet.gui._tags is never cleared, but new tags are added every time the content is updated.

This PR fixes this by:

• setting GuiGramplet._tags to an empty list in GuiGramplet.clear_text()
• calling GuiGramplet.clear_text() in GuiGramplet.set_text
• calling Gramplet.clear_text() in PedigreeGramplet.main()
  (Gramplet.clear_text() calls GuiGramplet.clear_text())

I couldn't find a bug report about this. I'm happy to create one if a discussion about this is preferred.

[Nick-Hall]

Did you intend this to target the gramps61 branch?

[ztlxltl]

I wasn't sure which version this bug should ideally be fixed in. I'm happy to change it if you prefer 6.1.

[Nick-Hall]

There will be no further releases in the gramps60 branch.