- from melody preview so far

- to refresh it after division or other transformations
- common method for check melody notes in instrument scale

- mark the melody preview with red if not
- and print appropriate text of this fact

- melody wrapper got preview button
- which invokes that preview component

- creates melody preview only when visible on the list
- is capable of reordering by dragging or from code

[Charts] Revert exam progress line functionality when hovered. Cleaned code: removed old 'graphic scene' lines, removed unused files.

Charts were rewritten to QML (in progress). Using ListView (ChartView.qml) with delegate (so far LinearDelegate.qml with C++ logic in TlinChartDelegate). So get rid of Qt graphics scene (a lot of code was removed or commented, some files are going to be deleted).
It dramatically reduces RAM usage (comparing to previous 1.5 alphas is hundreds of MB, comparing to 1.4 is plenty) - because only visible part of a chart are painted (rendered)

Revert bar charts, its tips as well. Display level preview for a chart exam, change chart window title

Restored charts functionality (partially) - QML Window with painted quick item rendering old chart scene
Moved common q/a texts to TnootkaQML

All external executables changed into libraries aka plugins to be loaded on damand. Placed in plugins folder in src/. Created single executable out of source dir to test plugins - launched with command line options