This commit fixes the initialization of the Locales instance in TYPO3 8,
because prior to version 9, the initialize method does not return the
initialized instance.

Fixes: #34

[FEATURE] Enable extension key whitelist

Closes #22

See merge request !28

Resolves: #22

[BUGFIX] Fix JS error when saving constant if main module name is Tools

Closes #32

See merge request !27

Fixes: #32

[BUGFIX] Fix compatibility with TYPO3 8.7

Closes #30

See merge request !26

The Environment class exists only since TYPO3 9, so we must introduce some
boilerplate code while TYPO3 8.7 is supported

Also, this commit fixes some misusages of PATH_site where paths were
already absolutes.

Fixes: #30

[BUGFIX] Clean of backend templates

See merge request typo3/lfeditor!25

- Optimize space taken by all the settings (less panels, more cols, responsive)
- TYPO3 styles for header bar and buttons
- Buttons' icon before the text
- Fix layout classes

[FEATURE] Allow to configure the position of LFEditor moduel in backend

Closes #28

See merge request typo3/lfeditor!24

Feature upgrade to9 lts

See merge request typo3/lfeditor!23

The l10n editing mode does not work properly with 9LTS. We are disabling
it for the time being.