enable static code checkers for gradle

With this pullrequest, gradle check task includes pmd, checkstyle and lint checks.

Due to the update of lint, we ignored some new lint checks, but some of these are fixed in this pullrequest.
Configs for lint, checkstyle and pmd are moved from catroid to a new folder in our root directory.

Additional information provided as comment in build.gradle file and in commit messages (especially for lint warning InflateParams).

After merging this pullrequest, some Jenkins configs have to be updated and developers have to rebase their branches for the custom-branch-tests.
Because of the config changes, I cannot provide a testrun of the latest commit, only a testrun of a previous commit: https://jenkins.catrob.at/view/All-Categories/view/Catroid-multi-job/job/Catroid-Multi-Job-Custom-Branch/1778/
However, there is a new Job on Jenkins, executing the gradle check task: https://jenkins.catrob.at/view/All-Categories/view/Util/job/GradleCheckTestingJob/3/