Making code compliant with GLPI v0.83.

List of required changes were found on GLPI Wiki (https://forge.indepnet.net/projects/plugins/wiki/En_Plugin080to083)
Merge branch 'migration-glpi_0.83' into develop
Making code compliant with GLPI v0.84.

The list of most required changes were found on GLPI Wiki (https://forge.indepnet.net/projects/plugins/wiki/Fr_Plugin083to084)
Others where found by watching PHP erors/notice while running plugin and analysing commits on GLPI project.
Fixed Html::header() usage: obsolete second parameter wasn't skipped.
Merge branch 'bugfix/1-no_submenu' into develop
Merge branch 'migration-glpi_0.84' into develop
CSRF compliance: updated forms and setup functions.
Merge branch 'evol/2-csrf_compliance' into develop
FIX #4: "PHP Notice: Undefined variable: result" when installing plugin

Made sure $result is correctly set as it should be.
FIX #5: "PHP Notice: Undefined index: field" when doing a mass action

Added missing "field" index for "Alternate username" criteria
Merge branch 'bugfix/5-undefined_index_field_when_massaction' into develop
Merge branch 'bugfix/4-undefined_result_when_installing' into develop
FIX #6: "PHP Notice: Undefined index: dropdown" when creating a new room type

Added missing translations.
Merge branch 'bugfix/6-undefined_index_dropdown' into develop
FIX #7:  Bad enconding in some files

Characters fixed.
Merge branch 'bugfix/7-bad_char_encoding' into develop
Reverting version changed by error

Version was changed in 688bc5ea55be8a64804131643af290775c78a4c9 and it shouldn't have.
Version 3.0.3: version for GLPI v0.84
Merge branch 'release-3.0.3'