Merge branch 'dev' into 'master'

Implement first versions of file browser, text viewer and image viewer

See merge request bohacfr2/apo-sem!1

feat: add files contextmenu

feat: add controls text file open

fix: execute program without argument

feat: add stderr dialog for executing file

fix: file opening throw error if nothing opened

fix: rework file opening mechanism so mime is checked first

feat: add file executing