Fix crash using Find Next or Find Previous

Fix crash of application when Find Next (F3) or Find Previous (Shift+F3)
actions were used without a previously performed search.

Fixes #88
Add french translation

Add french translation of application.
Thank you etienne-gauvin!

Resolves #89
Use pkgconfig for hunspell on unix

Use pkgconfig to find headers and libraries of hunspell on unix.

Issue #52
Update rpm spec file

Issue #52
Increase version to v0.8.1
Update README
Merge branch 'hotfix/v0.8.1'