Shortcuts taken to get something working sooner

[DinkydauSet]

• Keyboard shortcuts are not buffered. Pressing ctrl+T while the program hangs is handled as just pressing T.
• The scrollbars of the fractal part have a fixed width (vertical scrollbar) or height (horizontal scrollbar) to make calculations easier. It would be better to give them a weight, because nana is DPI-aware.
• A new tab has the wrong color for 1 frame and then immediately changes to the right color. More info: Creating a new tab is slow sometimes and I have no idea why #25 (comment)
• disabled the assert assert(ratio >= 0); in the function rgbColorAverage in common.cpp because it fails when a new render is started while an old render is being cancelled.
• These should ideally be as high as possible. I lowered the values for better performance. A very sensitive slider cancels bitmap renders all the time for even the slightest move, causing none to finish. This can be perceived by the user as bad performance. What is a better solution?

constexpr uint FINE_MAXIMUM = 250;
constexpr uint OFFSET_MAXIMUM = 250;