change qapp applicationName to lower case

Summary:
NB: this changes the standard config name of default constructed QSettings

(a quick grep seems to suggest there is no default constructed QSettings
 though, so this should be fine).

Aligning the applicationName with the spelling in the installation paths
on case sensitive systems allows us to use QStandardPaths' built-in
handling for application-specific paths instead of having to manually
append /falkon to various paths.

Test Plan: still seems to be working on linux

Reviewers: drosca

Reviewed By: drosca

Differential Revision: https://phabricator.kde.org/D7721