* fix: unsynchronized search index creation (code usage) results in ArrayIndexOutOfBoundsException and stuck at 99%

* fix: use computeIfAbsent instead of synchronized block

* feat: APK signature check v1/v2 using the apksig library from Google
* fix: proposed changes implemented

fix(gui): unsynchronized search index creation results in NullPointerException upon performing search (#429)

* fix: ArrayIndexOutOfBoundsException in string concatenation visitor
* fix: typo in comment
* fix: StringBuilder chain processing created wrong code
* test: simple JUnit test cases added for testing StringBuilder chain processing (chains that can be and that can't be simplified)

* fixed wait time for background jobs
* enable multi-threaded decompiling
* added preference for excluding certain packages from decompiling and indexing
* show message dialog in case classes are not indexed because of low memory
* added heap usage bar for visualizing Java memory usage