Install icons in the correct size-based folder

The following Lintian warnings were found while packaging everpad for Debian:
W: everpad: icon-size-and-directory-name-mismatch usr/share/icons/hicolor/64x64/apps/everpad-black.png 32x32
W: everpad: icon-size-and-directory-name-mismatch usr/share/icons/hicolor/64x64/apps/everpad-lens.png 128x128
W: everpad: icon-size-and-directory-name-mismatch usr/share/icons/hicolor/64x64/apps/everpad-mono.png 32x32