themechooser:  Load all the valid themes for the current version of myth,
including any tagged versions.  In other words, if the current versioin of
Myth is 0.27.3, then looks for themes for 0.27.3, 0.27.2, 0.27.1 and 0.27

(cherry picked from commit 0f6f7bd63659cafa526696ccad4d4062a80ef55a)
housekeeping: When checking for new theme versions consider all the tags.

(cherry picked from commit 57e085e49d9041235f01e28b77a06f3a0b5fd3a6)
ThemeChooser: Alert user when unable to modify files.

(cherry picked from commit 68f7fbd31097ff579a558aaf993d22072f6a0fcc)