Spectrum: Improvements of ZX "line composer"

[tomas-nestorovic]

Offering {K,L,E,G,C} modes, the line composer enables the user to create a line as they would on a real hardware. For instance, switching to the E mode and pressing the Z key, the LN keyword appears.

There are currently some imperfections to the CSpectrumBase::CLineComposerPropGridEditor class.

• Non-printable characters are not displayed. This is a major drawback as non-printable characters may appear in tweaked header files, particularly in name field. For instance, the name of the program of the Skull and Crossbones game is (hexa-decimally) 0x16, 0x01, 0x00, 0x53, 0x4B, 0x55, 0x4C, 0x4C, 0x06, 0x20. When loading, the name of the game looks as follows.
• Non-printable characters cannot be typed in. To solve for this, a new mode "X" should be introduced in which user can type in hexa-decimal numbers, which immediately translate to corresponding (printable) characters, UDG graphics, or keywords.
• The content doesn't scroll horizontally. It's thus impossible to edit the line "beyond" the visible rectangle. Instead, the line should be horizontally scrolled so that the cursor (and a bit of the line before and after it) is always visible.
• The cursors {K,L,E,G,C} are just a painting. It thus may lead to confusing situations in which there are suddenly two "cursors" on the screen - one in the line composer component and one in another component (e.g. hexa-editor). Instead, the cursor should be represented as a caret, so that it's guaranteed that at any moment there is just one "cursor" on the screen.
• The editor is mouse-unaware. When clicked into the line content, the ZX cursor should be positioned so that it's as close to the mouse cursor.

[tomas-nestorovic]

All problems mentioned in the issue description have been solved, and no new problems were observed.