Override pane title and pane title html tag should be separated

The ability to override a pane's title heading tag from an h2 to say an h3 was added in #2288747: Panels Title Headings and #2288741: Title Headings.

When I upgraded to the latest Panels release, I noticed all the pane's where I had overrided just the title now had the wrong heading tag. It was an h2 instead of h3.

I submitted a couple of patches for Panels and cTools at #2488280: Can't override the default Pane title heading tag to fix the problem where "H2" tags are hard coded for the default pane titles.

The patches in #2488280: Can't override the default Pane title heading tag allow me to override the default pane title tag from an H2 to an H3, yay! But.... With the new Panels 3.5+ when I edit or override a pane title, I have to specify a tag like h3 or h2. The problem is that each pane with an overridden title no longer uses the default pane heading tag.

So I'm suggesting to split "Override title" into two options:
-checkbox- Override title
-checkbox- Override title tag

That way if my theme ever changes to use H4 instead of H2 for the pane title tags, I only have to hunt down a few overridden panes.

With the patches in #2488280: Can't override the default Pane title heading tag, I just have to set a variable to override the default pane title heading tag.