Allow users to enable terminal cursor blinking with termux.properties

This `terminal-cursor-blink-rate` key can be used to enable terminal cursor blinking. The user can set an int value between `100` and `2000` which will be used as blink rate in millisecond. The default value is `0`, which disables cursor blinking. So adding an entry like `terminal-cursor-blink-rate=600` to `~/termux.properties` file will make the cursor attempt to blink every 600ms. Running `termux-reload-settings` command will also update the cursor blinking rate instantaneously if changed.

A background thread is used to control the blinking by toggling the cursor visibility and then invalidating the view every x milliseconds set. This will have a performance impact, so use wisely and at your own risk.

If the cursor itself is disabled, which is controlled by whether DECSET_BIT_CURSOR_ENABLED (DECSET 25, DECTCEM), then blinking will be automatically disabled. You can enable the cursor with `tput cnorm` or `echo -e '\e[?25h'` and disable it with `tput civis` or `echo -e '\e[?25l'`.

Note that you can also change the cursor color by adding `cursor` property to `~/colors.properties` file, like `cursor=#FFFFFF` for a white cursor.

The `TermuxPropertyConstants` class has been updated to `v0.9.0`. Check its Changelog sections for info on changes.

Closes #153

terminal-view/src/main/java/com/termux/view/TerminalRenderer.java

@@ -61,7 +61,7 @@ public final class TerminalRenderer {
         final int columns = mEmulator.mColumns;
         final int cursorCol = mEmulator.getCursorCol();
         final int cursorRow = mEmulator.getCursorRow();
-        final boolean cursorVisible = mEmulator.isShowingCursor();
+        final boolean cursorVisible = mEmulator.shouldCursorBeVisible();
         final TerminalBuffer screen = mEmulator.getScreen();
         final int[] palette = mEmulator.mColors.mCurrentColors;
         final int cursorShape = mEmulator.getCursorStyle();