Implemented variable overzoom with slider in "options/charts/vector char...

...ts" window.

This update is for Flyspray FS#1520
It adds a slider control to the "settings/charts/vector charts" window so the user can select the amount of overzoom they require. This modifies the amount of overzoom from the default "0" on the slider to more "10" on the slider. I have only checked this with CM93 charts as until I get cross compiling working I cannot check out the S63 charts.