Try to reduce flickering

[yubiuser]

What does this PR aim to accomplish?:

The major redesign of the way the dashboard is printed (#268) has introduced flickering on external displays (#290).

This PR is fixing this in two ways:

1. Use tput instead of stty to obtain the amount of rows and columns after a window resize. (The idea was to use stty because it has less overhead and is faster than tput).
2. The whole screen is not erased anymore before each refresh, but only when the window is resized.

Fixes #290 and #288

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