Terminal: keep the screen in the history when it is cleared

[insjang]

Erase in display (CSI 2 J) destroyed what was on the screen. It now moves it into the history first, the way Windows Terminal and VTE handle it, and then erases.

Why it matters. On Windows the local terminal runs through ConPTY, and ConPTY starts every connection with CSI 2 J followed by CSI H. Reconnecting a terminal therefore erased as many lines of the previous session as the screen was high: after seq 1 200 in a 31-line terminal, a reconnect kept only 1 to 171. The same happens whenever a program clears the screen, taking the user's last screen of output with it. This was captured from a real ConPTY session (wsl.exe) and replayed through the emulator; the loss is the same on current master.

Details:

• Blank lines at the bottom of the screen are not moved into the history, so clearing an empty or half-empty screen does not pad the history with blank lines.
• The cursor does not move, as ED requires.
• Where the buffer is at its maximum, the oldest lines make way. Where the buffer is capped at the screen height nothing is kept, so a screen with no history stays without one.
• CSI 3 J still clears the history, so clear (CSI H CSI 2 J CSI 3 J) empties everything as before.

xterm itself erases without keeping the screen; this follows the terminals most users on Windows and Linux compare Eclipse's terminal with.

Tests: testEraseAll now expects the screen in the history; testEraseAllKeepsTheScreenInHistory covers the blank lines at the bottom and the cursor, on both TerminalTextDataStore and TerminalTextData; testClearScreenKeepsItInHistory runs the sequence ConPTY sends through the emulator. All three fail without the change.