Rework filename input and overwrite check

[kirillt]

Filename input lags and keyboard acts strangely.

save_file.mp4

[kirillt]

When new file name coincides with one of existing files, there is no possibility to change it further. The app starts to think that user wants to override, even if the user just wanted to create several files with same prefix, e.g. test.jpg, test2.jpg, test3.jpg etc.