Implemented ved.inp.getKeyString(), to be used for in-game text keyboard input.

It retrieves keyboard info and outputs a string based on what the user has pressed. Should work regardless of keyboard layout, and supports characters like !@#$%^&*()_+<>?|
Fixing the comma, was originally a reversed version of "`"

How did I miss this?
Adding ved.gfx.Font.writeCentered() to center-justify every line of text.
Also centers along the y axis.
Fixed the bubbling issue in input.js. onkeypress is only used to capture text
but it does not get called unless onkeydown returns true. onkeydown returning
true also results in certain keys controlling the browser; end, home, etc.

Now onkeydown only returns true for keys associated with text characters.
Made it so the outermost tiles can't be changed.
Optimizing repulsor pod and bomb a little bit. Repulsor pod's physics can
get pretty expensive when it's acting on a lot of objects.
Tracking updownarrows.png for use with the listselect.
Removing the console.log that was used for debugging.
Made vedEngine maintain its own master clock so Date().getTime()
should only ever be called ONCE every time the main game loop runs.
Made the putTile functions return true if successful (the tile was
changed) or false if unsuccessful (the tile was already of the new
tile type, or it is an edge tile).
Tweaked the tractor beam so it can pull things through the air,
and made bombs work properly with the last change to map.js,
not generating rubble for edge tiles that don't actually get
destroyed.
Tweaked the player physics to be a little bit more "correct".

The player is now affected by gravity even when on the ground,
and the functions that need to know if he is on the ground now
check the ground variable instead of yvel.
Adding ved.Update() into the main loop to update the master clock.

ved.Update() also calls ved.inp.flushKeys() so removing that from
the main loop to prevent it being called twice.
Implemented a text input field that uses ved.inp.getKeyString(),
and responds correctly to delete, backspace, home, end, and the
arrow keys.

Added a text field demo to the start menu.

Made game_update.js get the time from ved.currenttime instead of
calling new Date().getTime().

Made game_draw.js draw the text on the pause screen with
writeCentered() instead of write(). Also changed it to use
the new big font instead of the 8x8 font.