Various dialer improvements

This is basically my take on the fix proposed by @sadiq here: https://source.puri.sm/Librem5/libhandy/merge_requests/109

Here is what it does:

• Merge the dialer digits and letters as a unified symbols concept, use the first symbol as the main dialer button label and the rest of the symbols as the secondary label.
• Don't make the secondary label smaller to better match the designs and make them dim on cycle buttons as the symbols are accessible.
• Move the + symbol on the 0 key to match the design of most dialers.