fix: replace recursive analysis algorithms with iterations to avoid StackOverflow on big methods (#441)

Fix visibility access modifies for methods (see discussions in #370 and #387):
    * all virtual methods become public
    * direct methods become private (instead constructors and static methods for now)
    * such modifications perform by default and can be disabled by the option in preferences (`--respect-bytecode-access-modifiers` in jadx-cli)
    * if changed to method added comment (`Access modifiers changed, original: private`)

fix(gui): instead gradle export was executed normal export
fix(gui): content of some resource files was not shown
perf: direct resource files saving without full length buffer in memory
perf(gui): line numbers will be disabled on big files due to performance issue
feat(gui): click on HeapUsageBar will run GC and update memory info
feat(gui): add more file types for syntax highlights
refactor: ResContainer class changed for support more types of data (added link to resource file)

* 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)