v0.11.7 enhancements

NOTE: This release requires the updated config database QtTSAprefs.db. It is backwards compatible with firmware older than 1.4.177 as far back as 1.4.120. It does not work with firmware 1.4.109 or earlier.

Much faster stopping on slow sweeps when 'stop' is pressed.
Changed the way the GUI screen is updated by the measurement thread. Previously it was updated every 20 measurements, so the update rate varied and for very fast sweeps could cause the GUI to freeze. Now it is a fixed rate and set in the 'preferences' dialogue in mS and there is a 'update rate indicator' at the bottom left of the main screen.
Changed measurement Fn to use the new scanraw auto-repeat option available with firmware 1.4.177 onwards.
Zero span now working as well as it can without re-coding
Added code for enhancement #58 (detect/choose from several connected tinySA, request by Herb Walker)
Added box to set the maximum scan frequency above 6GHz in 'preferences' dialogue (request by @ashjmckenzie )
Changed 'max' trace to permanent hold until scanning stopped/restarted, instead of a decaying max based on scan 'memory' (request by @CliveTombs)
Marker min/max detection frequency boundary lines added (request by @CliveTombs)
SyncTime, AverageBox & Sample repeat added to config database
Removed padding from the GUI display
Set Scan to Graph button (<>) added
Clicking a marker centres the sweep freq onto that marker
Units on freq axis now set automatically (changed preferences database to Hz instead of MHz)
Changes to openport Fn and testport Fn with error trapping

Bugfix: selecting a marker set freq incorrectly
Bugfix: band selection combobox didn't work unless tinySA hardware was detected
Bugfix: (#56) band add fail after preferences window opened
Bugfix: filebrowse picture not being shown if a csv was selected first
Bugfix: min/max marker Fn
Bugfix: when marker selected in band combo box

Thank you to @CliveTombs for a lot of testing.