Normalized font face and size so that the text looks similar in all environments

Key features

- Normalized font face and size so that the text looks similar in all environments. Previously some pieces of text were rendered with the default font which resulted in the text looking different depending on the Linux distribution used and./or the global preferences selected. Now the font has been explicitly set to Sans Serif family with a point size of 9 so that text is always rendered in a similar way. - Added instructions in the readme to build in Mint, openSUSE, Fedora and Manjaro

Other changes

- Added in the readme the references to the pre-built packages for openSUSE and Arch (Manjaro) - Added missing tool-tips in the various GUIs - NFM demod pushed volume by 2x - Added some missing dependencies in the readme (puseaudio is mandatory to run the program) - Added paragraph about code organization in the readme - Added frequency units for center frequency in all sample input GUIs